Screen printing or the like is used in pattern printing, solder resist printing, numbering printing, and through hole printing in a circuit forming process for a printed wiring board, and solder cream printing and exterior (chassis, name plate, operation panel) printing in a mounting process.
A typical printing machine for performing this screen printing performs printing by applying and transferring a pasty material, such as ink or solder paste, onto a printing surface of a printing material via a screen mask on which a print pattern is drawn. That is, a squeegee is slid on the screen mask, whereby the pasty material is transferred and printed on the printing material. Thus, the pasty material is applied on the printing material through an aperture pattern.
In this case, part of the pasty material remains at an aperture of the aperture pattern in the screen mask. This residual pasty material at the aperture leaks out from the aperture, and adheres to a back surface of the screen mask. This leakage occurs particularly when a pasty material having a low viscosity is used, and is a phenomenon in which the residual pasty material drops off from a wall of the aperture because of gravity and flows toward the back surface of the screen mask. If this adhering residual pasty material is transferred on the printing material, defects in the application position and application shape are caused, and in a circuit board, a quality defect, such as a bridge, is caused.
Further, in printing, if the adhesion between the screen mask and a substrate serving as a printing material is low, the pasty material exudes, and this causes a quality defect. FIG. 11 illustrates a substrate serving as a printing material. A plurality of mounted components 101 are mounted on one surface 100a of this substrate 100. Also, on the surface 100a and the other surface 100b of the substrate 100, there are provided copper foils 102 to which electrodes of the mounted components 101 are soldered, resist layers 103 serving as insulating layers, and silk ink layers 104 that are subjected to silk printing for indicating reference numbers or the like of the mounted components 101. When solder paste 105 is printed on this substrate 100 in a predetermined pattern, the substrate 100 is supported by backup pins 106 for preventing the substrate from warping. Then, a screen mask 107 is brought into tight contact with a printing surface (the other surface 100b), and a squeegee 108 is slid on this screen mask 107, whereby the solder paste 105 is transferred and printed. In this case, the surface of the substrate 100 is uneven because of the silk ink layers 105 and the like. When the solder paste 105 is applied on this surface via the screen mask 107, a gap of the order of microns is formed between the screen mask 107 and a printing position on the substrate 100. A phenomenon in which the solder paste 105 exudes into this gap is referred to as exudation. If printing is further performed in a state in which this exudation occurs, defects in application position and application shape and a quality defect, such as a bridge, are caused.
Accordingly, in the screen printing apparatus, it is necessary to timely clean the back surface of the screen mask.
As cleaning mechanisms for the back surface of the screen mask, there are a mechanism using a method in which cleaning paper for wiping the residual pasty material is used and a mechanism using a method in which the pasty material adhering to the back surface of the screen mask is scraped off with a scraper formed by a plate-shaped member, instead of wiping with the paper.
Incidentally, cleaning is necessary to realize high-quality printing. Also, there is a great demand to increase the printing tact. Accordingly, cleaning is performed in every several printing steps to increase the printing tact. This sacrifices quality.